Evidence has been accumulating that adenosine 3'5' cyclic monophosphate (cAMP) mediates the post-synaptic events associated with beta adrenoceptor stimulation of certain enzymes in pineal glands. Our studies on phosphodiesterases in bovine pineal glands showed the occurrence of multiple forms of this enzyme (PDE I and PDE II), having different molecular sizes, substrate specificity and kinetic parameters. One form (PDE II) was thermostable and had low molecular weight. PDE II could be purified to homogeniety by affinity chromatography on affinity gel blue column. An endogeneous peptide fraction in bovine pineal supernatant selectively inhibited PDE II whereas tetrahydrobiopterin strongly inhibited PDE I. Thus our results suggest that these two different forms of phosphodiesterases have differential endogeneous regulation as compared to other phosphodiesterases from various sources.